The present invention relates to the selective control of nerve fibers in the human nervous system. The invention is especially useful for selectively blocking sensory nerve fibers for blocking pain sensations while permitting other sensations to pass, and is therefore particularly described below with respect to this application. However, as will also be described below the invention could also be used in other applications, e.g., in selectively blocking motor nerve fibers for controlling muscles or glands.
The nervous system is a network of billions of interconnected nerve cells (neurons) that receive various types of stimuli and cause the body to respond appropriately. The individual nerve cells transmit the messages by means of a complicated electrochemical process which generates action potentials transmitted by axons, or nerve fibers. Such nerve fibers link the central nervous system (CNS) consisting of the brain and the spinal cord, with the body""s receptors and effectors in the peripheral nervous system (PNS). The receptors are sensory cells and organs responding to various types of stimulation, such as touch, pain, light, etc., which transmit action potentials through the sensory nerve fibers towards the CNS; whereas effectors are parts of the body, such as muscles and glands, that respond to instructions from the CNS transmitted via action potentials through motor nerve fibers to effect a particular activity in such muscles or glands.
Pain sensations have the useful purpose of alerting a person as to a condition, such as heat or cold, which can be injurious to the individual. However, there are many types of pains which do not serve this useful purpose and can cause severe discomfort or distress. For example, it is estimated that chronic pain partially or totally disables 50 million persons in the USA alone, and that 45% of the population seeks medical help for persistent pain at some point in their lives. Medical economists estimate that pain costs the U.S. some $100 billion every year, including 515 million workdays lost and 40 million doctors visits.
The most widely used controls for pain at the present time are narcotic treatments. The narcotics most commonly prescribed not only have a number of worrisome side effects, but are of limited effectiveness for millions of persons who suffer from neuropathic pain arising from damage to the nerves caused by disease, trauma or chemotherapy.
Some degree of pain control may also be effected by electrical stimulation. Current technology for pain control using electrical stimulation is based on the xe2x80x9cgate theory of painxe2x80x9d; see for example M. Devor, xe2x80x9cPain Networksxe2x80x9d, Handbook of Brain Theory and Neural Networks, Ed. M. A. Arbib, MIT Press, pp 698, 1998. This approach exploits the observation that pain sensations diminish when accompanied by a non-painful stimulus, such as the relief sensed when rubbing a painful area. Pain sensations are carried by the small-diameter nerve fibers (nociceptors), while normal sensations (such as touch) are carried by the large-diameter nerve fibers. To reduce pain, current techniques apply a low amplitude current which stimulates only the large-diameter fibers, since stimulation of the small-diameter fibers would induce pain. This is done in two ways: (a) Transcutaneous Electric Stimulation (TENS) by applying a small current externally to the skin; and (b) Dorals Column Stimulation (DCS), by inserting an electrode into the dorsal column and implanting a stimulating device nearby. This technique for pain control, however, has had a very limited degree of success.
A number of blocking techniques are also presently known for blocking or stimulating motor nerves controlling muscular or glandular activities. These include: (1) collision blocking; (2) high frequency blocking; and (3) anodal blocking.
In collision blocking, a unidirectional action potential is generated by external electrodes to travel towards the muscle or gland being controlled, i.e., from the CNS towards the PNS. These electrode-generated action potentials collide with, and thereby block, the body-generated action potentials.
In high frequency blocking, high frequency (e.g., 600 Hz) stimulations are used to block the transmission of the action potentials through the nerve fibers.
In anodal blocking, nerve fibers are locally hyper-polarized by anodal current. If sufficiently hyper-polarized, action potentials are not able to propagate through the hyper-polarized zone and will be blocked.
As will be described more particularly below, the anodal block has been investigated for producing a selective blocking of the action potentials through selected motor nerve fibers, particularly the larger-diameter nerve fibers which are more sensitive to the hyper-polarization. The unblocked electrode-generated action potentials (or those blocked to a lesser degree) passing through the anodal block are used, by collision blocking, for the selective control of motor nerve fibers in order to stimulate or suppress, as the case may be, selected muscular or glandular activities; see for example C. van den Honert, J. T. Mortimer xe2x80x9cA Technique for Collision Blocks of Peripheral Nerve: Single Stimulus Analysisxe2x80x9d, IEEE Transactions on Biomedical Engineering, Vol. 28, No. 5, pp 373, 1981, herein incorporated by reference.
The anodal blocking technique has been investigated for stimulating various motor nerves, e.g., for the restoration of bladder and urethral sphincter control, for skeletal muscle control, etc.; see for example D. M. Fitzpatrick et al., xe2x80x9cA Nerve Cuff Design for the Selective Activation and Blocking of Myelinated Nerve Fibersxe2x80x9d, Ann. Conf. of the IEEE Eng. in Medicine and Biology Soc., Vol. 13, No. 2, pp. 906, 1991, describing a tripolar electrode device useful for this purpose. Also see N. J. M. Rijkhof et al., xe2x80x9cAcute Animal Studies on the Use of Anodal Block to Reduce Urethral Resistance in Sacral Root Stimulationxe2x80x9d IEEE Transactions on Rehabilitation Engineering, Vol. 2, No. 2, pp. 92, 1994; N. J. M. Rijkhoff et al., xe2x80x9cOrderly Recruitment of Motoneurons in an Acute Rabbit Modelxe2x80x9d Ann. Conf. of the IEEE Eng., Medicine and Biology Soc., Vol. 20, No. 5, pp. 2564, 1998; and R. Bratta et al., xe2x80x9cOrderly Stimulation of Skeletal Muscle Motor Units with Tripolar Nerve Cuff Electrodexe2x80x9d, IEEE Transactions on Biomedical Engineering, Vol. 36, No. 8, pp. 836, 1989. The contents of the foregoing publications are incorporated herein by reference.
As described particularly in the above-cited Fitzpatrick et al publication, the tripolar electrode used for muscle control includes a central cathode flanked on its opposite sides by two anodes. The central cathode generates action potentials in the motor nerve fiber by cathodic stimulation; one anode produces a complete anodal block in one direction so that the action potential produced by the cathode is unidirectional; and the other anode produces a selective anodal block to permit passage of the action potential in the opposite direction through selected motor nerve fibers to produce the desired muscle stimulation or suppression. Further details concerning the construction and operation of such tripolar electrodes are set forth in the above-cited publications incorporated herein by reference.
An object of the present invention is to provide novel methods of selective control of nerve fibers which method is particularly useful for reducing pain sensations. Another object is to provide a method of selectrive control of nerve fibers which is also useful for controlling certain types of muscular or glandular activities. A further object of the invention is to provide apparatus for use in the above methods.
According to one aspect of the present invention, there is provided a method of reducing pain sensations resulting from the propagation of body-generated action potentials towards the central nervous system through small-diameter sensory fibers in a nerve bundle, without unduly reducing other sensations resulting from the propagation of body-generated action potentials towards the central nervous system through large-diameter sensory fibers in the nerve bundle, comprising: applying to the nerve bundle at least one electrode device capable, upon actuation, of generating unidirectional action potentials to be propagated through both the small-diameter and large-diameter sensory fibers in the nerve bundle away from the central nervous system; and actuating the electrode device to generate the unidirectional action potentials to produce collision blocks with respect to the body-generated action potentials propagated through the small-diameter fibers.
Preferably, this aspect of the invention utilizes the tripolar electrode devices described, for example, in the above-cited publications, except that, instead of using such tripolar electrodes for producing collision blocks of action potentials travelling through motor nerves away from the central nervous system in order to control muscular or glandular activity, there are used to produce collision blocks of action potentials propagated through sensory nerves towards the central nervous system in order to reduce pain sensations without unduly hindering other sensations.
According to another aspect of the present invention, there is provided a method of selectively suppressing the propagation of of body-generated action potentials propagated in a predetermined direction at a first velocity through a first group of nerve fibers in a nerve bundle without unduly suppressing the propagation of body-generated action potentials propagated in the predetermined direction at a different velocity through a second group of nerve fibers in the nerve bundle, comprising: applying a plurality of electrode devices to, and spaced along the length of, the nerve bundle, each electrode device beams capable of outputting, when actuated, unidirectional electrode-generated action potentials producing collision blocks with respect to the body-generated action potentials propagated through the second type of nerve fibers; and sequentially actuating the electrode devices with delays timed to the first velocity to produce a xe2x80x9cgreen wavexe2x80x9d of anodal blocks minimizing undesired blocking of the body-generated action potentials propagated through the first group of nerve fibers while maximizing the generation rate of said unidirectional electrode-generated action potentials producing collision blocks with respect to the body-generated action potentials propagated through said second type of nerve fibers.
Such a method may be used for producing collision blocks in sensory nerve fibers in order to suppress pain, and also in motor nerve fibers to suppress selected muscular or glandular activities.
According to a further aspect of the invention, there is provided a method of selectively controlling nerve fibers in a nerve bundle having fibers of different diameters propagating action potentials at velocities corresponding to their respective diameters, comprising: applying a plurality of electrode devices to, and spaced along the length of, the nerve bundle, each electrode device being capable of producing, when actuated, unidirectional electrode-generated action potentials; and sequentially actuating the electrode devices with delays timed to the velocity of propagation of action potentials through the fibers of one of the diameters.
In some described preferred embodiments, the electrode devices are sequentially actuated to generate unidirectional action potentials producing collision blocks of the body-generated action potentials propagated through the nerve fibers of another diameter. Such collision blocks may be used for suppressing pain sensations without unduly interfering with normal sensations, or for selectively suppressing certain motor controls without unduly interfering with others.
According to still further aspects of the present invention, there is provided apparatus for use in the above methods.
Further features and advantages of the invention will be apparent from the description below.